This invention related to thin film formation over substrates. More particularly, the invention relates to an interlayer thin film comprising a self-assembling monolayer (SAM) of monomers, one end of which are capable of bonding to the substrate, and the other end of which form a surface structurally similar to, and capable of initiating the formation of a material to be deposited. In a preferred embodiment, the invention relates to the initiation and bonding of thin diamond films to substrates.
2. Description of the Prior Art
The synthesis of hard thin coatings such as diamond film is an emerging technology with great commercial potential. The National Research Council, for example, published a monograph on the subject, Status and Applications of Diamond and Diamond-Like Materials: an Emerging Technology (National Research Council, Washington, D.C.) 1990, incorporated herein by reference. Organizations such as the Electrochemical Society, the International Society for Optical Engineering, and the Materials Research Society hold regular symposia on advances in the technology.
The target markets and opportunities for films of materials such as diamond are vast. Any object that requires ultimate scratch or wear protection, better slip or sliding properties, better heat removal or input, electrical insulation, or protection from the environment would benefit from a film coating that exhibits the properties of diamond. Coatings on magnetic or optical disks are two such possibilities. Since diamond films are also transparent to electromagnetic radiation with wavelengths between 225 nanometers (hard ultraviolet) and more than 40 microns, the optics in lasers and cameras could be protected from abrasion. A small sample of applications that could be implemented if the technology were available today, include without limitations: protection of windows, such as photocopier glass, bar-code readers, infrared detectors, plastic eyeglass lenses, automobile windows, aircraft canopies, optical fibers, and camera lenses; protection and lubrication of mechanical components, such as bearings, axles, and joints, including those for prosthetic use, and cutting or boring tools; and conductivity of heat in mechanical applications or from electronic components such as high-density semiconductor and integrated circuit chips. An article in the September, 1991 issue of Channel magazine projected the astonishing figure of forty billion dollars for annual sales of diamond products by the year 2000. However, this and other projections make assumptions about the rate of advancement of the technology. It is believed that only two diamond-film products have been commercialized to date: stereophonic tweeters and windows for certain X-ray instruments.
The difficulties that have impeded the commercialization of diamond coating can be classified according to two steps in the growth process: initiation or nucleation of the film, and growth of the film; and according to two properties of the grown film: crystallinity and adhesion.
In the prior art, film growth has been commonly initiated by scratching or rubbing a surface to be coated, or target surface, with diamond powder. Recent research has taken advantage of surfaces resembling diamond to initiate film formation. For example, layers of seed crystals have been deposited, using mechanical and physical means to orient them for use as diamond growth initiators, as disclosed in PCT Application 89 11,897, and in U.S. Pat. No. 5,082,359 incorporated herein by reference.
Molecular initiators are also known. In Applied Physics Letters, vol. 59, p. 3461 (1992), Meilunas et al have reported that a modified fullerene promotes diamond growth with good adherence to the substrate. U.S. Pat. No. 5,075,094 reports on the effects of pretreating substrate surfaces, both carbide-and non-carbide-forming, with diamond grit, evaporated carbon film, and hydrocarbon oils. Davis et al have proposed a study of organic interlayers, including oriented interlayers, as nucleators for diamond growth, on p. 23 of NTIS Report AD-A 244005, December 1991. Linolenic acid and polyimides, particularly the bis(N-phenylmaleimide)ketone, were cited as specific examples, but no attempt was made to orient them, and no suggestion advanced as to how orientation might be accomplished. Finally, Hotsuta et al disclose in Japanese Patent No. 01,103,991 the use of adamantane derivatives for modification of the substrate surface in order to control nucleation of CVD diamond. However, the modification requires immersion of the substrate in, e.g., an aqueous solution of 1-adamantylamine for 24 hours to allow equilibrium adsorption on the substrate. Each of the above references citing molecular initiation is incorporated herein by reference.
Following preparation of a surface to enable initiation, the growth of diamond film is usually carried out by passing a gaseous carbon source over or through an energy zone to decompose the gas into reactive particles. The NRC monograph describes several methods. For example, a stream of gas containing methane as the carbon source can be directed through an array of hot filaments or through a zone of microwave energy, where the methane will decompose into a mixture of atoms, ions and radicals, known as a plasma. Those reactive particles that possess the appropriate energy and orientation, and that strike the nucleated areas at suitable positions, deposit as part of a growing layer. Each of the individual nucleation sites begins to grow roughly hemispherically, until they all grow large enough to contact each other. At this point no room for lateral expansion remains, and all of the material that deposits contributes to increasing the thickness of the layer, at a rate that presently cannot be controlled uniformly. This process is termed chemical vapor deposition, or CVD.
Adhesion of diamond film to a substrate is often unsatisfactory because diamond film has a texture, in atomic dimensions, that usually does not match or fit the texture of the target surface; i.e., the diamond is heteroepitaxial. In some cases, such as on certain metals, the two textures match either perfectly or very closely, i.e., the diamond is homoepitaxial. In homoepitaxial conditions the diamond layer has enough contact points that adhere well to the substrate to provide overall good adhesion. In most cases, however, heteroepitaxial conditions apply, and the amount of contact is insufficient to provide satisfactory adhesion. Some of the research mentioned above in connection with initiation of film growth has attempted to address the adhesion problem. In addition, ion implantation has been studied as a method for promoting film adhesion. In this method, ions or ion clusters are fired into a substrate with sufficient energy to penetrate the surface, or melted in with a laser. By definition, they are fixed in the solid surface, and therefore any layer building on these implants will adhere to the surface in which they are embedded. However, ion implantation is a slow process, and the result is not a crystalline pattern resembling diamond on which a diamond layer can easily form and thicken.
As described in greater detail below, the invention utilizes self-assembling monolayer films. Organized SAMs have been known for many years, but their commercial potential has only recently been considered, for example in a review by Swalen et al in Langmuir, volume 3, p. 932 (1987). SAMs with two kinds of reactive groups, silyl and sulfur, at the end to be bonded with the substrate, have been the chief subjects of interest. The former has been recognized and described by Sagiv in U.S. Pat. No. 4,539,061 incorporated herein by reference. His monolayers are assembled chiefly from monomers bearing trichlorosilyl groups at the reactive end, and other groups with the capability of reacting further at the exposed end. The use of such SAM initiating layers to grow enantiomeric amino acid crystals has been disclosed by Landau et al in Nature, vol. 318, p. 353 (1985), but the application of such layers to diamond synthesis was apparently not considered. Other art has appeared using SAMs, also targeted at imaging. U.S. Pat. Nos. 4,908,299 and 4,968,524 both disclose systems similar to those of Sagiv, but bearing vinyl or acetylenic funtions at the exposed ends, which can then be selectively cross-linked to form a pattern. Disclosures of the use of SAMs for similar purposes have been made in U.S. Pat. Nos. 4,439,514 particularly example 23, 5,077,085, and 5,079,600. Each of the aforementioned references disclosing the composition and use of SAMs is incorporated herein by reference.
The sulfur-containing bonding ends have been studied extensively by Whitesides et. al., who reviewed their results in Langmuir, vol. 6, p. 87 (1990). Sulfur can react on surfaces which are inert to trichlorosilyl, for example gold. It is interesting that on page 938 of the review by Swalen et al, SAMs with tricholorosilyl and thiol reactive ends are discussed in the third paragraph, and diamond deposition in the fourth, yet in the five years elapsing since the publication of the review, apparently no one has applied SAMs to diamond film synthesis.
Diamond has thus far been the focus herein, because much of the literature on the problems discussed concern diamond. However, many other coatings, especially those with a crystalline, metallic, or alloy composition, require, or would benefit from the adhesive and/or initiating capabilities of the present invention. The field of ceramic coatings is particularly relevant. Indeed, a recent symposium of the MRS, published in the Materials Science Symposium Proceedings, vol. 221 (1991), was devoted exclusively to epitaxial ceramic coatings.